1. Technical Field
The present invention relates to a demodulator for demodulating digital information from an HMW-modulated or MSK-modulated wobble signal.
2. Related Art
A scheme for embedding digital information, such as address information, on an optical disk into a wobble signal of the optical disk has hitherto been adopted. In some optical disks typified by a Blu-ray disk, digital information is embedded through combined use of an MSK modulation scheme and an HMW modulation scheme. According to the MSK modulation scheme, frequency modulation is effected through use of two frequencies; namely, a frequency equal to the frequency of a reference carrier signal, and a frequency 1.5 times the frequency of the reference carrier signal. Consequently, provided that a reference carrier signal is represented as cos(ωt), binary digit “0” is represented by cos(ωt), or −cos(ωt), which is the inverse thereof. Binary digit “1” is represented by cos(1.5ωt), or −cos(1.5ωt), which is the inverse thereof. An MSK modulation mark is made up of three carrier periods cos(1.5ωt), −cos(ωt), and −cos(1.5ωt). According to the HMW modulation scheme, modulation is effected by means of adding or subtracting a harmonic signal of an even order (e.g., a second order) to or from a sinewave carrier signal in accordance with the sign of data to be modulated.
A plurality of techniques of demodulating digital information from the thus MSK-modulated or HMW-modulated wobble signal have hitherto been proposed. For instance, a demodulation scheme called a heterodyne scheme is described in Japanese Patent Laid-Open Publication No. 2003-123249. According to this scheme, when an HMW-modulated signal is demodulated, a harmonic signal of an even order and a data clock signal are generated from a reproduced wobble signal. The reproduced wobble signal is multiplied by the thus-generated harmonic signal of the even order, thereby acquiring a signal. The thus-acquired signal is summed on a per-data-clock-signal basis. A sign of address information is determined on the basis of a summed value acquired at the end edge of the data clock signal. When an MSK-modulated signal is demodulated, the wobble signal is multiplied by a reference clock signal in lieu of a harmonic signal.
However, the technique described in connection with Japanese Patent Laid-Open Publication No. 2003-123249 encounters a problem of insufficient reliability of a demodulation result. Specifically, in relation to a carrier signal, a harmonic component contained in a wobble signal is usually −12 dB or thereabouts. There may arise a case where a second harmonic component becomes unstable because of variations in manufacture of an optical disk or the influence of crosstalk from adjacent tracks, thereby posing difficulty in detection of a harmonic component and disabling demodulation of digital information. The same also applies to MSK modulation. According to the technique described in connection with Japanese Patent Laid-Open Publication No. 2003-123249, a demodulation circuit must be equipped with a multiplier, an integrator, or the like, and, hence, is liable to structural complication and cost hike.
Accordingly, the present invention provides a demodulator and an optical disk apparatus, which enable more accurate demodulation of digital information from an HMW-modulated or MSK-modulated wobble signal by means of a simple configuration.